Relays weren't the problem. It started up again today. I tried checking the low side pressure, but the compressor wouldn't kick in so I know the reading isn't correct.

I bought a can of r-134a with a gauge to check the levels, but the gauge doesn't return to zero after being disconnected so I'm not sure if it can be trusted once the problem goes away so the compressor will kick in.

You don't want to be arbitrarily dumping refrigerant in there. That system requires a very specific weight of refrigerant. If you put too much, you can destroy a $1,000 compressor and a lot more. That needs to be checked by a professional.

I too am having this problem started about a month ago. Mine seems to work fine unless its run for a couple hours and you stop for gas or something then when you retstart the car the ac will only blow hot air for thirty minuites or so. If I leavfe it off for thirty minutes or so it usually (so far) starts blowing cool air again.